[Diy_efi] Need help designing a circuit

[bcroe at juno.com]

31 Mar 04  Adam Wade <espresso_doppio at yahoo.com> writes:

> I'm trying to develop a small, solid-state device that
> will light a warning LED when one leg of a motorcycle
> stator start to become weak.

> The stator generates current in place of an
> alternator.  Most motorcycles us a 3-leg stator,
> making 3-phase AC between 8-80 V, then putting it
> through 6 diodes to rectify it, and to then feed the
> voltage regulator.

> I am looking to take a tap off of each of the 3 legs,
> and then to compare the AC voltages.  A difference of
> more than 1 V between each leg would trigger the lamp.
> 
> Is there a quick and easy way to build such a thing? 
> It would have to handle the full range of stator
> voltage.  Having the LED light when there was no
> voltage would also be handy, so one could see that it
> was operational at key-on.

If you can rectify the voltage off each phase to a representative 
DC voltage, this isn't hard.  A comparator (LM239) can detect 
when 2 voltages cross.  With 10V to power the '39, take a 1:15 
voltage divider into an input, then take a 15:(1 X .85) or 17.65:1 
divider into the other input of a comparator.  Maximum voltage 
the comparator sees is 80/15 = 5.33V (must be well below the 
'39 power).  If the voltage on the 15:1 drops below 85% of the 
voltage on the 17.65:1, the comparator will change state.  Build 
3 of these in a ring ('39 has 4 comparators), and you can pick 
up any weak phase.  

If one divider goes to a voltage instead of ground, you can make 
a voltage difference detector (instead of percentage) with all 
equal dividers.  Or make a combination of voltage and percentage.

Make the comparator output go low on fault; tie 4 comparator 
outputs together (open collector) so any one can signal fault.  
Use the 4th to detect no output before starting.  

Bruce Roe

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